1. Field of the Invention
This invention relates to process control systems. More specifically, the present invention relates to a process control system for controlling a plurality of devices of multiple different types, including both standard devices and nonstandard devices, using a standard control protocol.
2. Description of the Related Art
Present-day process control systems use instruments, control devices and communication systems to monitor and manipulate control elements, such as valves and switches, to maintain at selected target values one or more process variables, including temperature, pressure, flow and the like. The process variables are selected and controlled to achieve a desired process objective, such as attaining the safe and efficient operation of machines and equipment utilized in the process. Process control systems have widespread application in the automation of industrial processes such as the processes used in chemical, petroleum, and manufacturing industries, for example.
Control of the process is often implemented using microprocessor-based controllers, computers or workstations which monitor the process by sending and receiving commands and data to hardware devices to control either a particular aspect of the process or the entire process as a whole. The specific process control functions that are implemented by software programs in these microprocessors, computers or workstations may be individually designed, modified or changed through programming while requiring no modifications to the hardware. For example, an engineer might cause a program to be written to have the controller read a fluid level from a level sensor in a tank, compare the tank level with a predetermined desired level, and then open or close a feed valve based on whether the read level was lower or higher than the predetermined, desired level. The parameters are easily changed by displaying a selected view of the process and then by modifying the program using the selected view. The engineer typically would change parameters by displaying and modifying an engineer's view of the process.
Many process control systems include local field devices such as valves, motors, regulators and the like which are responsive to specific control protocols, such as Profibus, Fieldbus, CAN and the like, to implement various control function routines. Accordingly, these controllers are responsive to certain standard control protocols to implement control functionality in the field. The use of such standard control signal protocols can reduce the time and effort of developing a control system because a designer can use the same types of control signals from all devices responsive to the control protocol.
However, certain control devices are not responsive to standard control protocols. These devices are often responsive to other types of control signals such as digital ON/OFF signals, analog current signals or analog voltage signals. A system designer either has to avoid using field devices that are nonresponsive to an installed protocol, or develop systems that operate under one or more protocols. Thus, present day processing systems disadvantageously lack a capability to utilize both standard protocol control devices and devices that do not respond to control signals defined under the standard protocols.
What is needed is a process control system that controls both devices that are defined using a standard protocol and other, non-protocol devices in a manner that is transparent to the user of the process control system.